1. Field of the Invention
The present invention applies to the field of training sequences for radio communications systems and, in particular, to a training sequence having a unique periodic structure.
2. Description of the Prior Art
Mobile radio communications systems such as cellular voice and data radio systems typically have several base stations in different locations available for use by mobile or fixed user terminals, such as cellular telephones or wireless web devices. Each base station typically is assigned a set of frequencies or channels to use for communications with the user terminals. The channels are different from those of neighboring base stations in order to avoid interference between neighboring base stations. As a result, the user terminals can easily distinguish the transmissions received from one base station from the signals received from another. In addition, each base station can act independently in allocating and using the channel resources assigned to it.
Such radio communications systems typically include a broadcast channel (BCH). The BCH is broadcast to all user terminals whether they are registered on the network or not and informs the user terminals about the network. In order to access the network, a user terminal normally tunes to and listens to the BCH before accessing the network. It will then use the information in the BCH to request access to the network. Such a request typically results in an exchange of information about the network using separate control and access channels and ends in the user terminal receiving an assignment to a particular base station.
While frequency and timing offset can sometimes be determined by a user terminal based on the BCH, the initial request for access is typically received at the base station with an unknown amount of delay and unknown spatial parameters. In a spatial diversity multiple access system, the base station can enhance the capacity of the system by determining the position and range to the user terminal as well as any other spatial parameters. The delay in the arrival time of such request messages are proportional to the round trip delay encountered by messages traveling between the base station and the mobile terminal. For systems with a high coverage area per base station, this range and therefore the delay uncertainty may be very large. For example, a range of fifteen km results in a roundtrip delay time of around 100 microseconds.
In order to accurately resolve the access request and determine spatial parameters, a training sequence is typically transmitted with the request. Resolving the received signal using the training sequence can consume great computational resources and create delays in the base station""s response to the request. The greater the uncertainty of the received signal, the greater the computational resources that may be required.
A training sequence for a radio communications system is provided. According to one aspect of the present invention, the invention includes a core sequence of symbols, a successive repetition of the core sequence, and a marker sequence having a sequence of symbols different from the core sequence.